1. Field of the Invention
The present invention relates generally to holograms and the photosensitive hydrophilic gelatin layers which are present in holograms. More specifically, the invention relates to the susceptibility of gelatin layers to swelling caused by exposure to moisture.
2. Description of Related Art
Holograms are widely used for a variety of purposes including: helmet-mounded displays such as those disclosed in U.S. Pat. No. 3,928,108; eye protection reflectors for laser radiation; and novelty displays, such as pendant jewelry. In fabricating a hologram, a pre-holographic element comprising a hydrophilic, photosensitive layer on a substrate is processed to expose the photosensitive layer to an actinic interference pattern to record a latent image thereon. The photosensitive layer is then developed to obtain the recorded latent image.
The hydrophilic, photosensitive layers which are commonly used in holograms include photographic and holographic emulsions which utilize hydrophilic organic colloids as an emulsion vehicle. Dichromated gelatin, photographic silver halide emulsions, diazo gelatin and other photosensitive gelatins are examples of such materials. Typical thicknesses for the photosensitive layer is between about 1 micrometer to 100 micrometers. Layer thicknesses of 6-20 micrometers are most common.
A problem which has been experienced with photosensitive layers made from gelatins is that they are hydrophilic and tend to swell when subjected to moisture. Water is readily absorbed into the interstitial spaces present within the gelatin layer. The absorption of water causes the gelatin layer to swell and increase in thickness. This increase in layer thickness causes a shift in the wavelength. While this shift is more severe in reflection type holograms, it is present in all volume type holograms made by hydrophilic materials. If the wavelength shift is large enough, it will cause a spectral mismatch between the light source used to illuminate the hologram and the wavelength being reflected by the hologram. This is especially critical in applications where the spectral bandwidth of the source is very narrow, such as in the P-43 phosphor 5430 angstrom line, where the bandwidth is only about 40 angstroms wide. Further, the change in the gelatin layer thickness due to swelling and shrinking of the gelatin caused by changes in ambient moisture conditions can also produce continual undesired variations in the efficiency and quality of the hologram.
A cover or protective layer is placed over the photosensitive hydrophilic gel layer to protect the layer from damage due to abrasion and to prevent moisture from reaching the gelatin. The resulting hologram comprises a photosensitive gelatin layer which is sandwiched between a substrate and protective layer.
Plastic materials have been widely used as both the hologram substrate and protective layer due to their light weight, impact resistance and optical clarity. However, plastic materials are, by their nature, permeable to most gases, including water vapor. For example, polycarbonate is used as a substrate and cover for dichromated gelatin holograms in pilot's visors. The light weight, impact resistance and optical clarity of the polycarbonate makes it a desirable material for use in such pilot's visors. However, the inherent gas permeability of polycarbonate allows moisture to be transmitted through the polycarbonate within a few days thereby affecting the dichromated gelatin hologram.
Possible pilot's visors which have been suggested to eliminate moisture contamination of the gelatin hologram layer include visors made from glass, glass-plastic laminates and visors using a vapor deposited glass layer on the surface of the plastic. All of these proposed visors are open to further improvement. For example, glass alone is too heavy. Laminated structures in which thin layers of glass (approximately 0.003 inch) are laminated between plastic layers is acceptable for flat substrates but is difficult to work with when it must be spherically or toroidally formed. Further, cracking can occur upon minor impact.
The depositing of a layer of glassy or crystalline material onto the surface of a plastic visor in order to reduce permeability has shown some promise. These coatings are typically applied by various vapor deposition procedures, including sputtering and gas phase reactions. Since these coatings are all on the surface of the plastic, they are all subject to thermal stresses, abrasion and mechanical stresses. Problems have been experienced with these coatings in that they tend to fail under long exposure to high humidity and/or high temperatures. Further, it is difficult to vapor deposit glass as a layer on plastic without the formation of small pinholes.
Attempts to reduce the gas permeability of plastics are set forth in U.S. Pat. Nos. 4,318,970; 4,329,409; and 4,330,604. These patents disclose the use of crystalline and glassy deposited films and the use of thick layers of glass sandwiched between plastic. Also, prestressed plastics have been formed in which solid silicon particles are embedded in a plastic matrix and then subsequently oxidized by diffusing in oxygen gas. This results in a material with reduced permeability, however the material is not optically transparent. Also, the solid silicon has to be embedded in the plastic melt first with the subsequent treatment using oxygen gas to form SiO.sub.2 aggregates. These SiO.sub.2 aggregates are nearly the size of the silicon particles and not molecular in size. This results in a standard "filled" plastic which has the slight advantage of being stressed due to the increased size of the filler. However, the silicon particles are deposited randomly and therefore may not provide optimum reduction in permeability.
All of the above attempts to reduce the undesirable effects of moisture on hydrophilic gelatin holograms have involved preventing the moisture from contacting the gelatin. None of these solutions have been entirely satisfactory. Such glass and modified plastic materials having the desired gas impermeability are not suitable for many uses and they tend to be more elaborate and expensive than simple, single layer plastic substrates. Accordingly, there is a continuing need to find additional solutions to the problem of moisture absorption by hydrophilic gelatin holograms.